High-Speed Magnets: Exploring Faraday's Law and Lenz's Law
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Neodymium magnets are very strong. Adult supervision is recommended when using them. Do not let the magnets slam together. They may pinch your fingers or crack. Keep them away from small children, pets, credit cards, and pacemakers.
- Examine the equation form of Faraday's law, which states that the voltage generated in a coil of wire is proportional to the number of turns in the coil, and the rate of change of the magnetic flux. How can you manipulate either one of these two variables and measure the resulting change in voltage? Here are a few hints:
- The rate of change of magnetic flux depends on how fast the magnets are moving through the tube. Rather than shaking the generator (which would make the magnets' velocity difficult to measure), construct a longer tube and drop the magnets through it from a known height, which should allow you to calculate their velocity when they pass through the coil.
- In order to properly measure the voltage, you will need to use a multimeter set to measure AC voltage, or an oscilloscope. If you do not have access to either tool, you can use LEDs in series as a very rough measurement of the peak voltage (as described in the original Human-Powered Energy project idea). Keep in mind that voltages add in series, and the forward voltage drop of a red LED is roughly 2.2 V.
- Lenz's law describes how the polarity of the voltage induced in a coil depends on the direction a magnet's poles are facing and the direction in which the magnet is moving. Connect two LEDs to your coil with opposite polarities, meaning the LEDs should be wired in parallel, but facing opposite directions—the anode (long leg) of one LED should be connected to the cathode (short leg) of the other LED, and vice versa. How can you use this configuration to demonstrate Lenz's law? When you shake the generator, will the LEDs flash at the same time, or will they alternate? Here are a few hints:
- The timing of the LED flashes may be difficult to see with the naked eye. Many modern smartphones and digital cameras have a "slow motion" video capture option, which will record video at 60 or even 120 frames per second. Use this ability to film the LEDs as you shake the generator. Use a media player that lets you step through a video frame-by-frame to closely examine the timing of the LED flashes.
- Construct the tube for your wire coil using a transparency sheet (available at Amazon.com or an office supply store) instead of cardstock. This will allow you to see the magnet moving inside the coil. Rather than shaking the generator with your hand, which would be difficult to film, set up a longer tube and drop the magnet through it from a known height. Set up your camera so you can film the magnet falling through the tube and see the LEDs flashing in the same frame, as shown in Figure 1. This will allow you to correlate the position and velocity of the magnet within the tube with the timing and brightness of the LED flashes. Do your findings match up with your predictions based on Lenz's law?
Figure 1. This experimental setup has two LEDs connected with opposite polarity, and a generator with a clear tube so you can see the magnets moving inside.
To learn more about magnetic induction, Faraday's law, and Lenz's law, see the following website, or consult your high school physics textbook:
- Nave, C. (n.d.). Faraday's Law. Hyperphysics. Retrieved December 4, 2014.
Recommended Project Supplies
- Transparency sheets, available from Amazon.com
- Corrugated cardboard
- Craft knife or utility knife
- Super glue
- Smartphone or digital camera with slow-motion video capture mode
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